1. Field of the Invention
The present invention relates to a multi-core optical fiber, a multi-core optical fiber cable, and a multi-core optical fiber transmission system.
2. Related Background Art
Core twisting technologies are used to reduce polarization mode dispersion (PMD) in optical fibers configured to transmit light in a confined state inside a core extending along a predetermined axis. For example, Japanese Patent No. 2981088 (Patent Literature 1) shows a configuration wherein during fiber drawing of a single-core optical fiber, glass in a molten state is twisted (or spun) to randomize influence of residual stress due to non-circularity or non-circular symmetry of the core so as to leave no deviation in any specific direction, thereby to reduce the PMD caused by the fiber itself. International Publication WO2009/107667 (Patent Literature 2) shows a configuration wherein after glass solidifies, the single-core optical fiber is twisted to reduce the PMD due to outside factors of the fiber such as fiber bending or lateral pressure.
There are also studies on the core twisting technologies in multi-core optical fibers. For example, T. Hayashi et al., “Crosstalk Variation of Multi-Core Fibre due to Fibre Bend,” ECOC2010, We.8.F.6 (Non Patent Literature 1) shows that in an uncoupled multi-core optical fiber in which interference between signals is suppressed by reducing crosstalk between cores, the magnitude (statistical average) of crosstalk is independent of at least the twist of the optical fiber. Furthermore, J. M. Fini et al., “Crosstalk in multicore fibers with randomness: gradual drift vs. short-length variations,” Optics Express, vol. 20, no. 2, pp. 949-959 (Non Patent Literature 2) shows the simulation result indicative of a possibility of a significant reduction in inter-core crosstalk in an uncoupled multi-core optical fiber when the twist rate per unit length is precisely controlled and the fiber is twisted at a constant short pitch of about 100 rotations/m.